Single-layer MoS2 Research Articles

Effect of layer variation on the electronic structure of stacked MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy

Two-dimensional transition metal dichalcogenides (TMDCs) have the extensive application prospect in multifunctional electronics and photonics due to their unique electro-optical properties. In order to further expand their application scope in micro-nano optoelectronic devices and improve the performance of devices, the band-gap and defective engineering have been studied to tune the band-gap, morphology and structure of two-dimensional semiconductor materials. The tunning of the bandgap of MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy has been typically achieved by controlling the Se concentration. Theoretical calculations revealed that layered stacked two-dimensional alloy materials with a larger aspect ratio, exposed edges and obvious edge dangling bonds show enhanced HER activity as compared with TMDCs. In this paper, the properties of stacked MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy grown by the chemical vapor deposition method in a quartz tube furnace are investigated by using optical microscopy (OM), atomic force microscopy (AFM), scanning tunneling microscopy (SEM), Raman, photoluminescence (PL), and X-ray photoelectron spectroscopy (XPS). The OM and SEM images of the as-synthesized stacked MoS<sub>2(1-<i>x</i>)</sub>Se<sub>2<i>x</i></sub> alloy show apparent interface between layers and their thickness is further acquired by AFM. Unlike most of single-layer or few-layer MoS<sub>2(1-<i>x</i>)</sub>Se<sub>2<i>x</i></sub> alloys, stack-grown stepped MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy materials all present the strong luminescence properties despite the thickness increasing from 2.2 nm (~3 layers) to 5.6 nm (~7 layers). And even till 100 nm, the emission spectrum with two luminescence peaks can still be observed. The two exciton luminescence peaks A and B are derived from the valence band splitting caused by the spin-orbit coupling, respectively. As the thickness increases, the two luminescence peaks are red-shifted and exhibit a band-bending effect that is only present when the alloy doping concentration is changed. As the sample thickness is 5.6 nm, a C-peak at 650 nm at the high energy end of the PL spectrum is observed, which may be attributed to the transition luminescence from the defect energy level introduced by Se (S) substitution, interstice or cluster. When the number of layers is small, the number of defects is small, so that the luminescence is not observed. As the number of layers increases, the defects increase to form a defect energy level. However, when the material thickness continuously increases until the bulk material is formed, the luminescence disappears in the PL spectrum because the band gap is reduced and the band gap is made smaller than the defect energy level. Raman spectroscopy gives two sets of vibration modes:like-MoS<sub>2</sub> and like-MoSe<sub>2</sub>. The Raman peak is almost unchanged as the thickness increases, but the two vibration modes E<sub>2g (Mo-Se)</sub> and E<sub>2 g (Mo-S)</sub> in the plane gradually appear and increase. At the same time, the intensity ratio and line width of Mo-Se related vibration mode E<sub>2g</sub>/A<sub>1g</sub> increase with thickness increasing, which indicates the enhancement of the Mo-Se in-plane vibration mode and the incorporation of randomness of Se into the lattice. Obviously, the defects and stress are the main factors affecting the electronic structure of stacked MoS<sub>2(1-<i>x</i>)</sub> Se<sub>2<i>x</i></sub> alloy, which provides a meaningful reference for preparing the special functional devices and studying the controllable defect engineering.

Probing the nanoscale light emission properties of a CVD-grown MoS2 monolayer by tip-enhanced photoluminescence.

Two-dimensional transition metal dichalcogenides are gaining increasing interest due to their promising optical properties. In particular, molybdenum disulfide (MoS2) which displays a band-gap change from indirect at 1.29 eV for bulk materials to direct at 1.8 eV for the material monolayer. This particular effect can lead to a strong light interaction which can pave the way for a new approach to the next generation of visible light emitting devices. In this work we show the nanoscale variation of light emission properties by tip-enhanced photoluminescence microscopy and spectroscopy in the MoS2 monolayer, grown by chemical vapour deposition. The variations of the light emission properties are due to different effects depending on the shape of the MoS2 single layer, for instance, a different concentration of point defect in an irregularly shaped flake and the presence of a nanoscale terrace in a triangular monolayer. Simultaneously, atomic force microscopy reveals indeed the presence of a nanometric terrace, composed of an additional layer of MoS2, and tip-enhanced PL intensity imaging shows a localized intensity decrease.

A Physical Model for the Hysteresis in MoS2 Transistors

Even though the hysteresis in the gate transfer characteristics of two-dimensional (2D) transistors is a frequently encountered phenomenon, the physics behind it are up to now only barely understood, let alone modeled. Here, we demonstrate that the hysteresis phenomenon can be captured accurately by a previously established non-radiative multiphonon model describing charge capture and emission events in the surrounding dielectrics. The charge transfer model is embedded into a drift-diffusion based TCAD simulation environment, which was adapted to 2D devices. Our modeling setup was validated against measurement data on a back-gated single-layer MoS2 transistor with SiO2 as a gate dielectric. We use the modeling approach to gain a thorough understanding of the hysteresis, which will help to control this problem in future devices.

Highly active single-layer MoS2 catalysts synthesized by swift heavy ion irradiation.

Two-dimensional molybdenum-disulfide (MoS2) catalysts can achieve high catalytic activity for the hydrogen evolution reaction upon appropriate modification of their surface. The intrinsic inertness of the compound's basal planes can be overcome by either increasing the number of catalytically active edge sites or by enhancing the activity of the basal planes via a controlled creation of sulfur vacancies. Here, we report a novel method of activating the MoS2 surface using swift heavy ion irradiation. The creation of nanometer-scale structures by an ion beam, in combination with the partial sulfur depletion of the basal planes, leads to a large increase of the number of low-coordinated Mo atoms, which can form bonds with adsorbing species. This results in a decreased onset potential for hydrogen evolution, as well as in a significant enhancement of the electrochemical current density by over 160% as compared to an identical but non-irradiated MoS2 surface.

Gold Dispersion and Activation on the Basal Plane of Single-Layer MoS2

Gold islands are typically associated with high binding affinity to adsorbates and catalytic activity. Here we present the growth of dispersed nanoscale gold islands on single layer MoS2, prepared on an inert SiO2/Si support by chemical vapor deposition. This study offers a combination of growth process development, optical characterization, photoelectron spectroscopy at submicron spatial resolution, and advanced density functional theory modeling for detailed insight into the electronic interaction between gold and single-layer MoS2. In particular, we find the gold density of states in Au/MoS2/SiO2/Si to be far less well-defined than Au islands on other 2-dimensional materials such as graphene, for which we also provide data. We attribute this effect to the presence of heterogeneous Au adatom/MoS2-support interactions within the nanometer-scale gold cluster. Theory predicts that CO will exhibit adsorption energies in excess of 1 eV at the Au cluster edges, where the local density of states is dominated by Au 5dz2 symmetry.

Second harmonic generation property of monolayer TMDCs and its potential application in producing terahertz radiation.

Second harmonic generation (SHG) properties in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) have aroused great interest. However, until now SHG for TMDC monolayer alloys is seldom investigated. Meanwhile, there is considerable controversy over the static SHG coefficients of monolayer MoS2. The feasibility to produce terahertz (THz) radiation via SHG in pure and alloyed TMDCs has never been reported. We first calculate the SHG coefficients of monolayer MoS2, MoSe2, and MoS2(1-x)Se2x using the independent particle approximation plus scissors correction. We then simulate their THz absorption by applying density function perturbation theory plus the Lorentzian line and try to calculate their zero-frequency THz refractive index and birefringence. The physical property of MoS2(1-x)Se2x alloys is simulated by considering various combinations. Results indicate that monolayer MoS2, MoSe2, and MoS2(1-x)Se2x possess large static SHG coefficients and THz birefringence and display low absorption over broadband THz frequencies. Therefore, they have applications in producing THz radiation via SHG. This study demonstrates that THz radiation can be attained in a large number of monolayers and few-layers and will extend applications of 2D materials. Moreover, it is possible to identify the magnitude of static coefficients of single-layer MoS2 by measuring THz intensities.

Epitaxial Single-Layer MoS2 on GaN with Enhanced Valley Helicity.

Engineering the substrate of 2D transition metal dichalcogenides can couple the quasiparticle interaction between the 2D material and substrate, providing an additional route to realize conceptual quantum phenomena and novel device functionalities, such as realization of a 12-time increased valley spitting in single-layer WSe2 through the interfacial magnetic exchange field from a ferromagnetic EuS substrate, and band-to-band tunnel field-effect transistors with a subthreshold swing below 60 mV dec-1 at room temperature based on bilayer n-MoS2 and heavily doped p-germanium, etc. Here, it is demonstrated that epitaxially grown single-layer MoS2 on a lattice-matched GaN substrate, possessing a type-I band alignment, exhibits strong substrate-induced interactions. The phonons in GaN quickly dissipate the energy of photogenerated carriers through electron-phonon interaction, resulting in a short exciton lifetime in the MoS2 /GaN heterostructure. This interaction enables an enhanced valley helicity at room temperature (0.33 ± 0.05) observed in both steady-state and time-resolved circularly polarized photoluminescence measurements. The findings highlight the importance of substrate engineering for modulating the intrinsic valley carriers in ultrathin 2D materials and potentially open new paths for valleytronics and valley-optoelectronic device applications.

Improved Hysteresis and Reliability of MoS2Transistors With High-Quality CVD Growth and Al2O3Encapsulation

We report considerable improvement in the hysteresis and reliability of MoS2 field-effect transistors (FETs) achieved by chemical vapor deposition (CVD) of single-layer MoS2 and dielectric encapsulation. Our results show that a high-quality 15-nm thick Al2O3 layer allows for an efficient protection of the devices from adsorbent-type trapping sites. Combined use of the CVD-grown MoS2 as a channel and encapsulation simultaneously leads to at least an order of magnitude smaller hysteresis and up to two orders of magnitude lower long-term drifts of the transistor characteristics. Together with high on/off current ratios ( $\sim 10^{\textsf {9}}$ ) achieved in our devices, this presents a considerable advance in the technology of MoS2 FETs. As such, we conclude that both CVD growth of MoS2 and encapsulation present important technological steps toward reaching commercial quality standards of next-generation two-dimensional (2D) material technologies.

Friction characteristics of mechanically exfoliated and CVD-grown single-layer MoS2

In this work, the friction characteristics of single-layer MoS2 prepared with chemical vapor deposition (CVD) at three different temperatures were quantitatively investigated and compared to those of single-layer MoS2 prepared using mechanical exfoliation. The surface and crystalline qualities of the MoS2 specimens were characterized using an optical microscope, atomic force microscope (AFM), and Raman spectroscopy. The surfaces of the MoS2 specimens were generally flat and smooth. However, the Raman data showed that the crystalline qualities of CVD-grown single-layer MoS2 at 800 °C and 850 °C were relatively similar to those of mechanically exfoliated MoS2 whereas the crystalline quality of the CVD-grown single-layer MoS2 at 900 °C was lower. The CVD-grown single-layer MoS2 exhibited higher friction than mechanically exfoliated single-layer MoS2, which might be related to the crystalline imperfections in the CVD-grown MoS2. In addition, the friction of CVD-grown single-layer MoS2 increased as the CVD growth temperature increased. In terms of tribological properties, 800 °C was the optimal temperature for the CVD process used in this work. Furthermore, it was observed that the friction at the grain boundary was significantly larger than that at the grain, potentially due to defects at the grain boundary. This result indicates that the temperature used during CVD should be optimized considering the grain size to achieve low friction characteristics. The outcomes of this work will be useful for understanding the intrinsic friction characteristics of single-layer MoS2 and elucidating the feasibility of single-layer MoS2 as protective or lubricant layers for micro- and nano-devices.

Adsorption of DNA/RNA nucleobases onto single-layer MoS2 and Li-Doped MoS2: A dispersion-corrected DFT study

The kind of sensing platform in nano biosensor plays an important role in nucleic acid sequence detection. It has been demonstrated that graphene does not have an intrinsic band gap; therefore, transition metal dichalcogenides (TMDs) are desirable materials for electronic base detection. In the present work, a comparative study of the adsorption of the DNA/RNA nucleobases [Adenine (A), Cytosine (C) Guanine (G), Thymine (T) and Uracil (U)] onto the single-layer molybdenum disulfide (MoS2) and Li-doped MoS2 (Li-MoS2) as a sensing surfaces was investigated by using Dispersion-corrected Density Functional Theory (D-DFT) calculations and different measure of equilibrium distances, charge transfers and binding energies for the various nucleobases were calculated. The results revealed that the interactions between the nucleobases and the MoS2 can be strongly enhanced by introducing metal atom, due to significant charge transfer from the Li atom to the MoS2 when Lithium is placed on top of the MoS2. Furthermore, the binding energies of the five nucleobases were in the range of −0.734 to −0.816eV for MoS2 and −1.47 to −1.80eV for the Li-MoS2. Also, nucleobases were adsorbed onto MoS2 sheets via the van der Waals (vdW) force. This high affinity and the renewable properties of the biosensing platform demonstrated that Li-MoS2 nanosheet is biocompatible and suitable for nucleic acid analysis.

Remote N2 plasma treatment to deposit ultrathin high-k dielectric as tunneling contact layer for single-layer MoS2 MOSFET

Remote N2 plasma treatment is explored as a surface functionalization technique to deposit ultrathin high-k dielectric on single-layer MoS2. The ultrathin dielectric is used as a tunneling contact layer, which also serves as an interfacial layer below the gate region for fabricating top-gate MoS2 metal–oxide–semiconductor field-effect transistors (MOSFETs). The fabricated devices exhibited small hysteresis and mobility as high as 14 cm2·V−1·s−1. The contact resistance was significantly reduced, which resulted in the increase of drain current from 20 to 56 µA/µm. The contact resistance reduction can be attributed to the alleviated metal–MoS2 interface reaction and the preserved conductivity of MoS2 below the source/drain metal contact.

Ultradispersed and Single-Layered MoS2 Nanoflakes Strongly Coupled with Graphene: An Optimized Structure with High Kinetics for the Hydrogen Evolution Reaction.

As one of the most promising Pt alternatives for cost-effective hydrogen production, molybdenum disulfide (MoS2), although has been studied extensively to improve its electrocatalytic activity, suffers from scarce active sites, low conductivity, and lack of interaction with substrates. To this end, we anchor ultradispersed and single-layered MoS2 nanoflakes on graphene sheets via a hybrid intermediate (MoOx-cysteine-graphene oxide), which not only confines the subsequent growth of MoS2 on the graphene surface but also ensures the intimate interaction between Mo species and graphene at the initial stage. Mo-O-C bond and a possible residual MoO3-x layer are proposed to comprise the interface bridging the two inherent incompatible phases, MoS2 and graphene. This strongly coupled structure together with the highly exposed MoS2 morphology accelerates the electron injection from graphene to the active sites of MoS2, and thus the hydrogen evolution reaction (HER) can achieve an overpotential of ∼275 mV at ∼-740 mA cm-2, and a Pt-like Tafel slope of ∼35 mV dec-1. Our results shed light on the indispensable role of interfacial interaction within semiconducting material-nanocarbon composites and provide a new insight into the actual activity of MoS2 toward the HER.

Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

Few- and single-layer MoS2 host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS2-a natural one and one prepared at high pressure and high temperature-and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS2. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials.

Physical Model for Rapid and Accurate Determination of Nanopore Size via Conductance Measurement.

Nanopores have been explored for various biochemical and nanoparticle analyses, primarily via characterizing the ionic current through the pores. At present, however, size determination for solid-state nanopores is experimentally tedious and theoretically unaccountable. Here, we establish a physical model by introducing an effective transport length, Leff, that measures, for a symmetric nanopore, twice the distance from the center of the nanopore where the electric field is the highest to the point along the nanopore axis where the electric field falls to e-1 of this maximum. By [Formula: see text], a simple expression S0 = f (G, σ, h, β) is derived to algebraically correlate minimum nanopore cross-section area S0 to nanopore conductance G, electrolyte conductivity σ, and membrane thickness h with β to denote pore shape that is determined by the pore fabrication technique. The model agrees excellently with experimental results for nanopores in graphene, single-layer MoS2, and ultrathin SiNx films. The generality of the model is verified by applying it to micrometer-size pores.

Interfacial thermal resistance between few-layer MoS2 and silica substrates: A molecular dynamics study

Interfacial thermal resistance (ITR) between molybdenum disulfide (MoS2) and crystalline or amorphous silica as a substrate was studied using molecular dynamics (MD) simulation. To do so, pump–probe method, which is a MD technique inspired by an experimental method, was employed. The effects of substrate type, temperature, number of layers of MoS2 and van der Waals (vdW) coupling strength on ITR between MoS2 and its silica substrates were explored. It was observed that, obtained ITR values for crystalline or amorphous silica substrate were close to one another. Our findings showed that, by increasing the temperature from 200 K to 400 K, ITR between a single-layer MoS2 and its crystalline or amorphous silica substrate decreases by about 20%, which might be due to better phonons couplings at the interface in higher temperatures. We also showed that, ITR between multilayer MoS2 and crystalline or amorphous silica substrate does not differ by increasing the number of layers of MoS2. It has been found that, by increasing Lennard-Jones coupling strength from 0.5 to 2, ITR between a single-layer MoS2 and crystalline/amorphous silica substrate decreases by around 80%, showing better phonons couplings at the interface between the two structures.

On the nature of AFM tip metal-MoS2 contact; effect of single layer character and tip force

In the present study, the nature of metal-MoS2 contacts has been investigated using conductive atomic force microscopy. The point I-V characteristics have been acquired as a function of loading force (53 nN–252 nN) for an interface formed between the atomic force microscopy (AFM) tip (Pt and Co coated) and MoS2 domains (single and multilayer). The metal-MoS2 junctions for both Co and Pt tips exhibit a rectifying nature with higher forward current for the Co tip as compared to the Pt tip for both single and multilayer samples. An interesting behavior of decreasing junction current and rectification ratio with increasing tip loading force is observed in the case of single layer MoS2. In comparison, negligible or very small change in the forward junction current is observed in the case of multilayer samples. The single layer MoS2 undergoes charge polarization in the presence of uniaxial strain exerted in the form of AFM tip loading force, resulting in current in a direction opposite to the forward current due to the migration of majority electron carriers away from the tip-MoS2 interface. Therefore, the net current which is a sum of the forward junction current and the force induced current reduces with loading force for single layer MoS2. This study emphasizes the influence of the single layer nature of MoS2 on the electrical properties of metal-MoS2 contacts, an understanding of which is important from the perspective of basic physics and its applications.

Nonvolatile ferroelectric memory based on PbTiO3 gated single-layer MoS2 field-effect transistor

We fabricated ferroelectric non-volatile random access memory (FeRAM) based on a field effect transistor (FET) consisting of a monolayer MoS2 channel and a ferroelectric PbTiO3 (PTO) thin film of gate insulator. An epitaxial PTO thin film was deposited on a Nb-doped SrTiO3 (Nb:STO) substrate via pulsed laser deposition. A monolayer MoS2 sheet was exfoliated from a bulk crystal and transferred to the surface of the PTO/Nb:STO. Structural and surface properties of the PTO thin film were characterized by X-ray diffraction and atomic force microscopy, respectively. Raman spectroscopy analysis was performed to identify the single-layer MoS2 sheet on the PTO/Nb:STO. We obtained mobility value (327 cm2/V·s) of the MoS2 channel at room temperature. The MoS2-PTO FeRAM FET showed a wide memory window with 17 kΩ of resistance variation which was attributed to high remnant polarization of the epitaxially grown PTO thin film. According to the fatigue resistance test for the FeRAM FET, however, the resistance states gradually varied during the switching cycles of 109.

Semiconductors: Temperature‐Triggered Sulfur Vacancy Evolution in Monolayer MoS2/Graphene Heterostructures (Small 40/2017)

In article number 1602967, Yanfeng Zhang, and Zhongfan Liu, and co-workers demonstrate that sulfur vacancies generated in MoS2/graphene heterostructures via a temperature-triggered process, lead to a decrease of the bandgap and n-doping effect of single layer MoS2.

Controllable Growth of Single Layer MoS2 and Resistance Switching Effect in Polymer/MoS2 Structure

Controllable Growth of Single Layer MoS₂ and Resistance Switching Effect in Polymer/MoS₂ Structure

Band Alignment at Au/MoS2 Contacts: Thickness Dependence of Exfoliated Flakes

We investigated the surface potential (Vsurf) of exfoliated MoS2 flakes on bare and Au-coated SiO2/Si substrates using Kelvin probe force microscopy. The Vsurf of MoS2 single layers was larger on the Au-coated substrates than on the bare substrates; our theoretical calculations indicate that this may be caused by the formation of a larger electric dipole at the MoS2/Au interface leading to a modified band alignment. Vsurf decreased as the thickness of the flakes increased until reaching the bulk value at a thickness of ∼20 nm (∼30 layers) on the bare and ∼80 nm (∼120 layers) on the Au-coated substrates, respectively. This thickness dependence of Vsurf was attributed to electrostatic screening in the MoS2 layers. Thus, a difference in the thickness at which the bulk Vsurf appeared suggests that the underlying substrate has an effect on the electric-field screening length of the MoS2 flakes. This work provides important insights to help understand and control the electrical properties of metal/MoS2 contacts.